This application relates to and incorporates by reference Japanese patent application number 2001-215655, which was filed on Jul. 16, 2001.
The present invention relates to a capacitive humidity sensor that has a humidity-sensitive film between a pair of electrodes. The capacitance of the film changes in response to humidity, and the sensor detects humidity by detecting changes in capacitance between the electrodes in correspondence with changes in the surrounding humidity.
A conventional capacitive humidity sensor, in general, includes a semiconductor substrate; a pair of electrodes, which are formed on a surface of the semiconductor substrate and face each other across a distance; and a humidity-sensitive film placed between the electrodes and formed on a surface of the semiconductor substrate. The capacitance of the film changes in response to humidity. The sensor detects humidity by detecting changes in capacitance between the pair of electrodes in response to changes in the surrounding humidity.
Such a capacitive humidity sensor is provided, for example, in Japanese examined patent publication JP-B2-6-105235 and Japanese unexamined patent publications JP-A-55-66749 and JP-A-60-166854. The sensors of these publications include a lower electrode formed on a substrate, a humidity-sensitive film placed on top of the lower electrode, and a thin, humidity-permeating upper electrode on top of the humidity-sensitive film. These sensors are susceptible to problems with reliability and resistance to humidity, however, because the upper electrode is exposed to the outside environment.
The present invention addresses these issues by providing a capacitive humidity sensor that converts changes in capacitance between a pair of detection electrodes, in response to changes in humidity, into voltage signals using the SC circuits for detection. A first objective of the invention is to keep the sensor small. A second objective is to ensure electrode reliability by preventing the detection electrodes from being exposed to the outside environment.
To achieve these objectives, the invention includes a capacitive humidity sensor, which includes a semiconductor substrate; circuit devices formed on a surface of the semiconductor substrate; a pair of detection electrodes, which are formed on the surface of the semiconductor and oppose each other across a distance; and a humidity-sensitive film, the capacitance of which changes in response to humidity, located between the detection electrodes and formed on the surface of the semiconductor substrate. The capacitance between the detection electrodes changes in response to changes in the surrounding humidity. The circuit devices include switched capacitor circuits having a reference capacitor and a feedback capacitor for converting changes in capacitance between the detection electrodes into voltage signal output. At least one of the reference capacitor and feedback capacitor is an underlying capacitor that is placed beneath a detection electrode.